Firstly, commiserations for the obvious hassle you've had to endure over what should have been a straightforward transaction.
It's very unfortunate to say the least that your Police Officer brother has seemingly put you off by telling you that there's nothing the Police can do!
I disagree for the following reason(s):
1. Someone else is allegedly in possession of your property, they must be aware of that fact that the ring isn't theirs, therefore there's a fair chance that a criminal offence, i.e. theft, may have been committed. Just ask your brother the Law's definition of theft and it should become clearer.
2. The jeweller's should have CCTV, as others have mentioned, therefore if they claim to have released it to someone within the past week or so, you would expect them to have some kind of record of who was given the ring, plus at least the date.
3. If you haven't already done so, consider telling the jeweller that you're reporting the matter as theft to the Police and see if this elicits a positive response.
4. The jeweller had a duty of care whilst the ring was in their possession, unless they have any waivers drawing your attention to same, which means that they have failed in that duty.
Don't let them off lightly - demand satisfaction for the loss of a treasured item which was none of your doing.
Best of luck.